1. Field of the Invention
The present invention relates generally to fluid reaction surfaces, and more specifically to attaching a turbine stator vane with blade to platform attachment structure.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine includes a turbine section with four stages of stator vanes and rotor blades that convert the energy from the hot gas flow into mechanical energy that drives the rotor shaft. The engine efficiency can be increased by passing a higher temperature flow into the turbine. The highest temperature safely capable of use is limited to the material characteristics of the turbine components, especially the first stage vanes and blades since these airfoils are exposed to the directly discharged from the combustor.
An airfoil made from a single crystal material can be operated under a higher temperature than a nickel based super-alloy. However, a single crystal material vane is difficult to cast because the platforms for the vane are also cast with the airfoil as a single piece. A lower successful cast rate is accomplished with single crystal vanes, which significantly increases the overall cost for a stator vane. Nickel super-alloy vanes are cast as a single piece with the outer shroud or platform used to support the vane in the engine. The inner shroud or platform of the vane is located on the opposite end of the vane and produces a seal between the rotor blade and shaft. Thus, the load placed on the stator vane by the passing hot gas flow is supported totally by the outer shroud or platform of the stator vane. Therefore, the outer shroud of the stator vane must be massive and rigid enough to support the vane during engine operations.
Therefore, there is a need in the art for a stator vane that includes an airfoil portion made from a single crystal material, and for an un-coupled platform that can be made from a different material but secured to the airfoil portion so that the outer shroud or platform can adequately support the loading on the vane.
One prior art blade attachment method is shown in U.S. Pat. No. 5,129,786 issued to Gustafson on Jul. 14, 1992 and entitled VARIABLE PITCH FAN BLADE RETENTION ARRANGEMENT which discloses a fan blade attached to a disc arm by circular shaped pins secured within first and second seating grooves formed in the blade root and the disc arm opening. One problem with the Gustafson invention is that the circular retaining pins cannot withstand very high shear stress that would result in a turbomachine such as a compressor that operates at high rotational speeds. Another problem with the Gustafson invention is that the resulting force of the fluid acting on the surface of the blade will cause the blade root portion to bend within the supporting opening in the disc arm. In the Gustafson invention, because the retaining pins do not follow the outline of the airfoil surface, the airfoil bending load does not transfer directly to the shear pin.
Another prior art blade retaining method is shown in U.S. Pat. No. 2,974,924 issued to Rankin on Mar. 14, 1961 and entitled TURBINE BUCKET RETAINING MEANS AND SEALING ASSEMBLY which discloses a turbine blade (bucket) attached to the rotor disk by pins fitted within slots on the sides of the blade and the opening of the rotor disk. Four pins for each blade are used, with two pins on each side of the blade root, and where the two pins on the side are angled or offset along a straight line from each other. This offset arrangement of the retaining pins will support the shear loads from the bending force acting on the airfoil surface more than in the above cited Gustafson invention, but still not like the present invention. also, Rankin discloses the retaining pins to be circular or round in cross sectional shape, but also discloses that the pins can have a square cross section (see column 2, line 60).
It is an object of the present invention to provide for a stator vane with an airfoil made from a single crystal material.
It is another object of the present invention to provide for a stator vane with the shrouds un-coupled from the airfoil portions, but capable of supporting the stator vane under engine operations.
It is another object of the present invention to provide for a stator vane in which the airfoil portion can be easily replaced in the stator vane assembly (the airfoil and the platforms).